Image detecting systems typically scan scene based images onto a focal plane array made up of individual detector elements. Each detector element within the focal plane array has a unique output voltage offset and gain reported upon detection of an image. The unique output voltage offsets and gains of the individual detector elements are a result of an inherent inability to produce identical detector elements during the manufacturing process. The inability to produce identical detector elements requires that an imaging system perform calibration on the focal plane array to compensate for the varying output voltage offsets and gains from one of the individual detector elements to another despite detecting identical portions of an image.
Calibration of imaging systems typically requires that calibration reference levels be viewed by the focal plane array during system operation. The calibration time can take time away from viewing the actual scene images. The calibration references of typical scan systems are produced by thermo-electrical devices that are optically viewed during the calibration period. Replacing the thermo-electrical devices with modified optical elements already in the system is desirable to reduce system complexity. Viewing these modified optical elements during normal scan return times also simplifies system operation.
Therefore, it is desirable to perform calibration with modified scene elements inherent within a typical scan system.
From the foregoing, it may be appreciated that a need has arisen for scene based calibration of an image detecting system. In accordance with the present invention, a passive scene based calibration system is provided that substantially eliminates or reduces disadvantages and problems associated with conventional image detecting system calibration techniques.
According to an embodiment of the present invention, there is provided a passive scene based calibration system that includes an optical environmental interface for receiving image energy from a scene. The image energy from the scene passes through the optical environmental interface to an afocal lens assembly that performs an image forming function and a calibration function in response to the image energy. A scanning mirror scans the image energy across a focal plane array. The focal plane array forms an opto-electronic approximation of the infrared detail of the image energy. A converging optics assembly inverts the image energy prior to reaching the focal plane array.
The present invention provides various technical advantages over conventional image detecting system calibration techniques. For example, one technical advantage is to perform scene based calibration. Another technical advantage is to perform calibration using the same lens configuration as used in image detecting. Yet another technical advantage is using a field lens that has an inner portion for image detection and an outer portion for calibration. Other technical advantages are readily apparent to one skilled in the art from the following figures, description, and claims.